Scanning probe microscopes (SPMs) can be used to obtain extremely detailed analyses of the topographical or other features of a surface, with sensitivities extending down to the scale of individual atoms and molecules. SPMs basically scan a probe over a sample surface and make local measurements of the properties of the sample surface. Several components are common to practically all scanning probe microscopes. The essential component of the microscope is a tiny probe positioned in very close proximity to a sample surface and providing a measurement of its topography or some other physical parameter, with a resolution that is determined primarily by the shape of the tip and its proximity to the surface. In a scanning force microscope (SFM), the probe includes a tip which projects from the end of a cantilever. Typically, the tip is very sharp to achieve maximum lateral resolution by confining the force interaction to the end of the tip.
One common example of an SPM is the atomic force microscope (AFM), also known as the scanning force microscope (SFM). By measuring motion, position or angle of the free end of the cantilever, many properties of a surface may be determined including surface topography, local adhesion, friction, elasticity, the presence of magnetic or electric fields, etc. In operation, an AFM typically will scan the tip of the probe over the sample while keeping the force of the tip on the surface constant, such as by moving either the base of the lever or the sample upward or downward to maintain deflection of the lever portion of the probe constant. Therefore, the topography of a sample may be obtained from data on such vertical motion to construct three dimensional images of the surface topography.
It is also known that AFMs utilize analog and digital feedback circuits to vary the height of the tip of the probe or the sample based upon the deflection of the lever portion of the probe as an input. An image may be formed by scanning a sample with respect to the probe in a raster pattern, recording data at successive points in the scan, and displaying the data on a video display. The development of atomic/scanning force microscopy is described in articles by G. Binnig at al., Europhys. Lett., Vol. 3, p. 1281 (1987), and 1. R. Albrecht et al., J. Vac. Sci. Technology, A6, p. 271 (1988). The development of the cantilever for AFMs is described in an article by T. R. Albrecht at al., entitled “Microfabricated Cantilever Stylus for Atomic Force Microscopy”. J. Vac. Sci. Technol., A8, p. 3386 (1990).
Other types of SPMs, such as scanning capacitance or scanning magnetic force microscopes, also use similar deflection sensors. Moreover, scanning tunneling microscope (STM) is similar to an SFM in overall structure and purpose, except that the probe consists of a sharpened conductive needle-like tip rather than a cantilever. The surface to be mapped must generally be conductive or semiconductive. The metallic needle is typically positioned a few Angstroms above the surface. When a bias voltage is applied between the tip and the sample, a tunneling current flows between the tip and the surface. The tunneling current is exponentially sensitive to the spacing between the tip and the surface and thus provides a representation of the spacing. The variations in the tunneling current in an STM are therefore analogous to the deflection of the cantilever in an SFM. The head contains circuitry for biasing the tip with respect to the sample and preamplifying the tunneling current before it is passed to a controller. Further details of SPMs are described in U.S. Pat. Nos. 5,025,658 and 5,224,376, the entire disclosures of which are incorporated herein by reference.
DIP PEN NANOLITHOGRAPHY™ (a trademark of Nanoink, Inc.) printing, also referred to as DPN (also a trademark of Nanoink, Inc.) printing, is conceptually the nano-version of the 4,000-year-old quill pen. DPN printing, which can be performed using an SPM, is a direct-write lithography technique based upon the transport of materials from a nanoscopic tip onto a surface of interest (e.g., paper). DPN printing allows one to draw fine lines or patterns one molecule high and a few dozen molecules wide.
In one embodiment of DPN printing, an AFM tip is coated with a patterning compound (also referred to herein as an “ink”), and the coated tip is contacted with the substrate so that the patterning compound is applied by capillary transport to the substrate to produce a desired pattern in submicrometer dimensions. Chemisorption can be used as the driving force for patterning ink onto the paper, as the tip is scanned across this paper. Through DPN printing, line widths can be controlled by adjusting scan rate and relative humidity. The relative humidity controls the size of the meniscus between the AFM tip and surface of interest and, therefore, the effective contact area between pen and paper.
By way of example of DPN printing methodology, an oily “ink” of octadecanethiol (ODT) is applied uniformly to an AFM's tip. When the tip is brought into contact with a thin sheet of a gold substrate or “paper,” the ODT molecules are transferred to the gold's surface via a tiny water droplet that forms naturally at the tip. Other details of DPN printing methodology are described in International Patent Application No. PCT/US00/0031 9, the entire disclosure of which (including defined terms contained therein) is incorporated herein by reference.
When using multiple inks or patterning compounds with DPN printing, probe tips of different patterning compounds sometimes need to be interchanged or even replaced during the process, as needed. Also, the sample sometimes needs to be removed for an intermediate processing step. As a result, finding a previously deposited patterning compound on a sample surface requires cumbersome and imprecise manual techniques. One such technique involves the use of plastic sheets that are taped onto a CRT screen showing an image of the sample surface. Markers are then used to manually mark up the plastic sheet to essentially designate positional coordinates of the relevant objects depicted on the CRT screen. In order to work with a sample that was previously made, or when changing patterning compounds, a DPN printing experimenter must utilize the plastic sheet template that was created in a correct orientation. As can be appreciated, the manual nature of marking coordinates to align multi-ink patterns is tedious and inconvenient, increases the chance for misalignment errors, and increases the time and effort needed to perform multi-ink DPN printing. Thus, there remains a need for more efficient methods and apparatuses that allow multi-ink patterning to be performed with more than three or four inks without the attendant disadvantages of conventional methods and apparatuses.